1. Field of the Invention
The present invention relates to a steam turbine nuclear power plant and more particularly to the control of LP turbine cycle steam and metal temperatures by governing the heat transfer operation in a moisture separator-reheater.
2. Description of the Prior Art
The temperatures and pressures of the cycle steam in the main steam header (MSH) of a nuclear steam supply system (NSSS) are considerably lower than that of fossil steam systems. The energy in the steam supplied to the high pressure (HP) turbine in a nuclear plant is just about enough to evaporate water. Theoretical heat rate calculations, have determined that at approximately 35% of rated turbine cycle steam flow, saturated (wet) steam conditions will exist at the exhaust of the HP turbine. Since the cycle steam is further needed to transmit energy to the one or more downstream low pressure (LP) turbines, a moisture separator-reheater (MSR) unit is provided as part of the nuclear steam turbine supply package to remove the moisture content and reheat the HP exhaust steam to dry conditions. Heating steam is usually supplied to the reheater from a hot steam supply source, generally the MSH. Typically, the heat transfer control of the MSR's has been performed by governing the flow of the heating steam through tube bundles located within the MSR in accordance with a set of control modes. One such MSR controller presently in use is described in U.S. Pat. No. 3,898,842, "Electric Power Plant System And Method For Operating A Steam Turbine Especially Of The Nuclear Type With Electronic Reheat Control Of A Cycle Steam Reheater" by M. Luongo, issued Aug. 12, 1975, which is referred to herein for a better understanding of MSR control and nuclear turbine cycle steam operational limitations.
In the control of reheating cycle steam, the operational limitations of the downstream LP turbine must be considered. For example, protection must be provided to guard against rapid heating or cooling of turbine parts in order to avoid excessive distortion and thermal-fatigue cracking thereof. In addition, at low loads, it becomes necessary to reduce the steam temperature at the LP turbine inlet to protect against overheating and subsequent overstressing the last two rows of rotating LP turbine blading. The MSR controller described in the previously mentioned U.S. Pat. No. 3,898,842 provides a set of operating modes for protection of the downstream LP turbine according to the foregoing criteria. The operating of such controller modes may be selected by a power plant operator or a programmed power plant computer in accordance with the state of the turbine process as exhibited to either the operator or the plant computer. Certain sequencing of operational modes entails knowledge of prior and existing turbine states. As a result, judgment of the turbine conditions and decision in determining control operations are left entirely to the operator or the plant computer. In either case, the MSR operational mode selection is most likely to occur in time concurrent with other equally necessary decision making control functions, thus burdening the responsible party. It is one object of the invention, then, to alleviate the burdens of MSR control by providing a MSR controller which functions autonomously.
To better appreciate the control of the MSR, one must understand the operation of the surrounding process effecting such a control. Heat transfer within a typical MSR is based primarily on temperature and flow of the heating steam and the temperature and flow of the cycle steam. The heating steam flow to one such MSR is governed typically by a control valve as a function of a generated reference temperature set point. It is preferred in MSR control that the temperature at the LP turbine inlet increase linearly as a function of load during load ramp conditions. However, in an NSSS, the MSH temperature and likewise pressure will vary mainly as a function of cycle steam flow. To further complicate matters, the cycle steam temperature at the HP turbine exhaust will also vary with cycle steam flow. In the above referenced MSR controller, one mode of operation attempts to linearly increase the LP turbine steam inlet temperature by ramping the generated reference temperature set point at 100.degree. F./hour There is provided no closed-loop control of LP turbine steam inlet temperature during the temperature ramp process. As a result, the variations in the upstream turbine process temperatures and pressures may cause undesirable temperature changes in the LP turbine. It is apparent then to improve the protection of the LP turbine elements against possible thermal stresses and distortion due to excessive temperature changes, an MSR control based on measured LP turbine inlet steam temperature is needed. It is another object of the invention to provide a closed-loop control function of the LP turbine steam inlet temperature across the turbine load spectrum. It is further apparent that LP turbine inlet steam temperature may not always provide enough information concerning LP turbine stationary and rotating blade metal temperatures considering the variation in the upstream turbine process parameters. Therefore, it is a further object of the invention to provide control functions based also on LP metal temperatures to improve the protection of the LP turbine elements.
In order to improve availability figures by reducing the downtime of nuclear turbines, the LP turbine steam inlet is provided at the sides of the LP turbine and below the horizontal joint to allow access to any of the LP turbine elements without prior disassembling of the conventional crossover piping and interceptor valves and without disturbing any other element. Side steam inlets also reduce erection time by permitting installation of crossover piping while rotors are being aligned. To reduce the amount of crossunder and crossover piping and simplify the piping arrangements to conform with the side entry LP turbines, two MSR's are provided and positioned on each side of the LP turbines. Each MSR conducts steam flow from one HP exhaust line through its heating chambers to one side of one or more LP turbines. Inlet steam to the LP turbines is not mixed prior to entering the LP turbine admission arc, thereby permitting a possible temperature differential across the admission arc. This temperature differential causes uneven thermal expansion in stationary and rotating LP turbine elements. Should these conditions become excessive and remain uncontrolled, it is possible for deleterious effects to result therefrom. Therefore, it is another object of this invention to provide an MSR controller to control the temperature differential across the LP admission arc within the operational limitations of the LP turbines.